Perception does not depend on sensation

The MindHacks blog had a link to a interview of Nicholas Humphrey by Alan Saunders in Philosophers Zone (here). There are a number of interesting observations in the recording. Here is one about the separation of perception and sensation.

…The essential point to make is that seeing, hearing or touch or smelling for that matter, is not a single dimension, it hasn’t got just one dimension to it. It’s terribly important to distinguish perception from sensation. The perception of red means understanding, representing the effect about the world, that there is a traffic light out there and that it has a particular coloured light coming from it, or to take another example, that there’s a tree standing in the forest and that it’s quite a particular shape and position and so on. Those are facts about the world, and we use our eyes, use other senses, to get that kind of other information.

Sensation, by contrast, isn’t about what’s out there in the world, it’s about our own response to the stimulation falling on our sense organs… These two things, sensation and perception, are often confused and it’s been traditional in philosophy and certainly I think it’s most people’s view that actually perception depends on sensation. We only get to know that the traffic light is red because first of all we have a sensation. …

What I try to do in the beginning of the book is to show that actually and amazingly this isn’t the case. Perception proceeds independently of sensation; we don’t have to have the sensation in order to get to know what’s out there in the world. And I go through a lot of examples to make that case. Perhaps the most remarkable is the phenomenon of blind sight. It’s something I guess I was responsible for discovering many years ago when I was working, not with human beings but with monkeys.

As a student in Cambridge I had the opportunity to study a monkey who had the visual cortex at the back of her brain removed. My supervisor had done the experiment. He had established that this monkey was apparently quite blind … I had the chance to spend some time with this monkey, she was called Helen, and over literally a few days, sitting with her, playing with her, it became clear to me that she wasn’t actually as blind as she was meant to be. … But I realised there was something very strange about her vision. There must have been something wrong with her you could have guessed, because she didn’t have any visual cortex to see with, but what I became convinced of was that she didn’t believe that she could see. She didn’t seem to have the evidence in front of her eyes if I can put it like that, that she could see. Although every day she would demonstrate objectively that she could. She would run, climb a tree, she would pick up a bit of chocolate from the floor or whatever it may be, she’d come up and take my hand. Out of that work I began to speculate about the possibility of unconscious vision, visual perception occurring without the possibility, without I should say, the fact of sensation. So the subject wouldn’t understand why they could see or have any reasons to believe that they could see.

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